1. Field of the Invention
This invention relates to a method and device for increasing the energy efficiency of a power plant. More specifically it relates to a method and a device which optimizes the back pressure in an air-cooled condenser to more efficiently regulate the power consumed in operating an air-cooled condenser.
2. Background Art
Some power plants create electricity by burning fuel to create intense heat. The heat is used to vaporize liquid water running in pipes near the heat source such as a boiler 10 to generate steam. The steam inside the pipes is under great pressure and is directed to pass over and drive the blades of a turbine generator 12. The steam forces the turbine generator to spin thereby creating electricity. See FIG. 1.
Some of the electricity created by such a power plant is used to operate subsystems of the power plant itself. Power plants can have numerous subsystems in their designs such as boilers, steam condensers, offices and shops. While some of the subsystems consume a portion of the electricity the power plant creates, many are designed to increase the power plant's net electrical output. Power plant subsystems which operate using electricity from the plant, such as an air-cooled condenser 14, have an optimum level or set point at which the subsystem functions with the greatest energy efficiency under various conditions.
Achieving optimal energy efficiency in a subsystem and maintaining it are longstanding problems. In order to monitor and evaluate subsystem energy efficiency, the electrical consumption of the subsystem and the electrical output of the generators are simultaneously monitored and evaluated in relation to each other. If the subsystem is not running efficiently, appropriate adjustments should be made. It is advantageous if the subsystem being monitored provides means for adjusting the power consumption to various levels. If the subsystem being monitored provides only limited means for adjusting power consumption, it is more difficult to run the subsystem efficiently. Subsystems for which power consumption can be readily and selectively adjusted and varied can be operated more efficiently.
An air-cooled condenser 14 of a power plant is a subsystem that consumes plant energy and that increases the power plant's net electrical output by its design and efficient operation. Air-cooled condensers cool steam after the steam has been used to drive the generators, thereby condensing the steam into liquid water and conducting it back to the heat source where it is used again. As steam leaves the turbine generator, the steam is still under pressure and is conducted into condensing tubes 16 where the steam is allowed to cool and to condense. In order to take advantage of natural cooling, condensers and condensing tubes are typically placed out-of-doors, or in a location where the surrounding ambient air, wind and weather can be utilized to cool the condensing tubes. The speed of the condensing process can be increased by lowering the temperature of the condensing tubes through which the steam flows. In an air-cooled condenser, large industrial fans 18 can be positioned near the condensing tubes to create additional air flow over the condensing tubes to lower the temperature of condensing tubes. See FIG. 2. Running the fan motors of the large condenser fans accounts for most of the electricity consumed by the condenser subsystem. Because of the significant amount of electricity used to operate condensers, great efforts are made to make the condensers energy efficient. Select air-cooled condensers are configured with means for adjusting and varying power consumption of the condenser. However, if the condensing subsystem does not operate efficiently, excess energy is consumed, thereby reducing the plant's net electrical output.
The main parts of a typical air-cooled condenser system include a main steam header, steam condensing tubes, large fans, water pipes and an air ejector system. The main header is a conduit that carries the steam from the generator turbines to the steam condensing tubes. Once the steam is in the condensing tubes, the condensing tubes, and consequently the steam inside the condensing tubes, are cooled by the large fans so that the desired amount of steam condenses again into liquid water. Water pipes connected to the base of the condensing tubes collect the condensed water and conduct it back to the power plant's heat source. An air ejector system (AES) 20 evacuates uncondensed gases from the condensing tubes and ejects them from the condenser subsystem. See FIG. 1.
Using a condenser to condense steam exhausted from the generator turbine has a significant limitation. If the steam is not sufficiently condensed and/or uncondensed gases are not ejected quickly enough, then as the turbine continues to exhaust steam, pressure on the exhaust side of the turbine builds. This pressure, called back pressure, can work against the turbine actually causing a resistance to the desired operation of the turbine, decreasing the work output, thereby decreasing the electrical output of the generator and the plant.
To help regulate back pressure build up, the back pressure in the system is monitored and regulated in prior art techniques by physically measuring actual back pressure and comparing it to a preselected, calculated value for back pressure, called the back pressure set point. The back pressure set point can be selected by the operator. The back pressure set point regulates back pressure by changing the rate of cooling and condensing in the condenser subsystem by controlling the condenser fan speed. Condenser back pressure is monitored and when the actual back pressure is lower than the desired, predetermined set point, condenser fan speed is lowered, allowing back pressure to increase. Conversely, when the actual back pressure exceeds the desired, predetermined back pressure set point, the condenser fan speed is increased to accelerate cooling and condensing of the steam, thereby reducing back pressure. However, increasing the fan speed also increases electrical consumption which may reduce the net electrical output of the plant. A longstanding problem resides in attempting to optimize back pressure so the turbine generators, condensers and fans are operating at optimum energy efficiencies.
Some condensers use fans having variable speed fan motors which allow the fan speed to be adjusted very precisely. Other condensers may use fan motors which provide one or only a few discrete settings for fan speed. Without a variable speed motor on the fan, the fine adjustments to the fan speed necessary for the fans to run most efficiently are more difficult to make. Variable speed fans allow the operator to adjust the fan speed precisely for the most efficient use of the fan. The variable speed fans are able to efficiently cool the condensing tubes so the steam in each tube cools as it rises up through the length of the tube.
Even with variable speed fans, an air-cooled condenser is operated inefficiently when more energy is used for fan speed increase than is gained by reducing back pressure. For example, if the back pressure is significantly limiting generator output, an operator may choose to reduce the back pressure in order to produce more electricity. Back pressure is reduced by increasing the fan speed of the condenser fans. However, running the fans at higher speeds consumes more electricity. At some point the extra energy gained by reducing back pressure is equal to the extra energy consumed by running the fans at higher speeds. Beyond that point, reducing back pressure any further by increasing fan speeds actually decreases the net electrical output because the fans use more electricity than they save by reducing back pressure. In other words, there are conditions when running fans at higher speeds consume more energy than is created by the corresponding reduction in back pressure.
Similarly if the amount of steam flowing into and driving the turbine is increased to increase generator output, the condenser subsystem cooling rate may have to be increased to condense the additional steam entering the condenser subsystem. After the generator reaches its optimal net output, any incremental gain in generator output is offset by an equal power loss or consumption required to operate the condenser subsystem at the rate needed. When the incremental power increase to operate the condenser subsystem exceeds the incremental increase in generator output, the generator is no longer operating in an efficient range and net power output actually decreases with each increase in gross generator output.
In order to increase the efficiency of a condenser subsystem, the generator output of the plant and the power consumed by the condenser subsystem are directly measured, monitored, compared, and adjusted by prior art techniques to run the subsystem more efficiently. If monitoring and comparing actual generator output to power consumption shows the subsystem is consuming more power than it saves, then adjustments are made to increase the subsystem's efficiency.
In attempting to monitor generator output and the power consumption, it is well known that the readings for these conditions can fluctuate frequently. The source of the fluctuations may be conditions such as existing, ambient temperature and wind conditions around the condenser(s). When ambient temperatures are low and wind speeds are high the condenser functions differently than when temperatures are high and wind speeds are low. Thus, in order to monitor generator output and power consumption the fluctuations in the actual generator output and the subsystem power consumption can be monitored and compared to help regulate the efficiency of the subsystem. More specifically, the range of the fluctuation of the actual generator output may be compared to the range of the fluctuation of the power consumption.
Monitoring the actual generator output and comparing it to the subsystem power consumption offers some indication of the efficiency of the power consumption by the subsystem, but monitoring actual generator output has a significant disadvantage. Monitoring the actual generator output provides a generator output reading that is also influenced by uncontrollable external factors such as boiler temperature, pre-exhaust steam quantity and flow rates of various other subsystems within the power plant. These external factors affecting actual generator output value are not necessarily related to the efficiency of controllable subsystems. When external factors influence the value used to determine generator output, they compromise the ability to accurately monitor the effect of the controllable subsystem on generator output. Depending on the subsystem being monitored, other external factors might also include the main steam pressure, the reheat spray flows, boiler conditions, and changes in the heat transfer. For example, the external factors causing variations in generator output may be wholly unrelated to the power consumption of the condenser subsystem being monitored for efficiency. External factors make it difficult to determine whether fluctuations in the generator output are related to fluctuations in condenser efficiency. Therefore, monitoring actual generator output is not the most reliable method for evaluating condenser efficiency.
What is needed is a method for monitoring generator output relative to the power consumption of a controllable subsystem such as the condensing subsystem which method more effectively isolates the monitoring of generator output from external factors which are not related to the subsystem efficiency. The isolated generator output value can then be compared to the subsystem power consumption, and the subsystem can be regulated accordingly.